The Unsorted BBS Collection/thegreatunsorted.tar/thegreatunsorted/programming/asm_programming/a05.doc

Text File | 1994-01-18 | 11KB | 250 lines

CHAPTER 5   SOME EXCLUSIVE FEATURES OF A86


The IF Statement

As a "nudge" in the direction of structured programming, A86
offers the IF statement.  Suppose you want to conditionally skip
around just one instruction.  Ordinarily, this would require, for
example:

  JNZ >L1    ; skip the following move if NZ
  MOV AX,BX  ; make this move only if Z
L1:          ; this label exists only for the above skip

You may replace the above code with the single line:

  IF Z MOV AX,BX

The above line generates exactly the same code as the previous 3
lines-- a conditional jump of the opposite condition, around the
statement given in the tail of the IF statement.  The statement
can be a macro call, giving you the opportunity to skip something
more complicated.

You may use any condition that would follow the "J" in a
conditional jump instruction, except CXZ, which does not have a
reverse condition.  The assembler interprets the condition by
appending a "J" to the beginning of the condition; so that the
symbols "C", "NC", "Z", "NZ", etc. are not reserved by the
assembler, and can be defined in other contexts.


Multiple operands to PUSH, POP, INC, DEC

A86 will accept any number of register operands for the
instructions PUSH, POP, INC, and DEC; it will generate the
appropriate machine instruction for each operand.  For example,
the statement PUSH AX,BX is the same as the two statements PUSH
AX and PUSH BX.

A numeric operand appearing in an INC or DEC statement will cause
the previous INC(s) or DEC(s) to be propagated that number of
times.  For example, the statement INC AX,4 will generate 4 INC
AX instructions.  The statement DEC AL,BX,2 will generate DEC AL,
DEC BX, DEC AL, DEC BX.  Sorry, numeric operands are not allowed
if any of the operands affected was a forward reference or
relocatable quantity; e.g., INC FOO,2 where FOO is undefined.  In
most such cases, you'll want to code the more efficient ADD FOO,2
anyway.


Repeat Counts to String Instructions

A86 will accept a numeric operand to the string instructions
STOSB, STOSW, MOVSB, and MOVSW.  This causes A86 to generate that
many copies of the given instruction.  For small values (usually
2 through 4), this is more efficient than loading the number into
CX and using the REP prefix.
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Conditional Return Instructions

Programmers accustomed to the conditional return instructions of
the 8080/Z80 will appreciate the following feature: A86 allows
the operand to a conditional jump instruction to be one of the
three RET instructions RET, RETF, or IRET.  The assembler will
find a nearby return instruction of the indicated flavor, and use
that as the target for the conditional jump.  For example, JZ RET
is the replacement for the 8080's RZ return-if-zero instruction.
In other 8086 assembly languages, you have to find the nearby
instruction yourself, attach a label to it, and use that label.
Note that it does not suffice to attach a label to a single RET
instruction and use that label throughout the program: the range
of conditional jumps is only 128 bytes in either direction.

What happens if A86 does not find a nearby return instruction? In
that case, A86 issues an error, "02 Jump > 128", for the next
matching return instruction in the program.  If there is no
subsequent return instruction, the return mnemonic will appear as
an undefined symbol at the end of the program.   In either case,
you correct the problem by inserting a free-standing return
instruction at some nearby point in the program, where it will
not affect the existing code (typically following an
unconditional JMP instruction).  If there is no good place to
insert a return instruction, you can always replace the "Jcond
RET" with an "IF cond RET".


A86 extensions to the MOV and XCHG instructions

There are a number of MOV and XCHG instructions available in A86
that are not a part of the machine instruction set.

First, moves between segment registers, and of immediate
constants into segment registers are allowed.  For example, if
you code MOV ES,DS , the assembler will generate a PUSH DS
followed by a POP ES; which will effect the move that you
intended.   If you code MOV DS,0 , the assembler will generate
PUSH AX; MOV AX,0; MOV DS,AX; POP AX.  This is mainly a
convenience for D86 users to load segment registers manually.

Second, MOV allows 3 operands.  A statement MOV x,y,z is
equivalent to the two statements MOV y,z followed by MOV x,y.
Sorry, but segment overrides are not allowed in conjunction with
3-operand MOVs.  The override preceding the MOV is ambiguous in
its meaning; and overrides within operands cannot be handled
correctly by A86.  You'll have to code two MOV instructions if
you want either or both to have a segment override.

Third, A86 accepts a MOV of a word-sized memory operand into
another word-sized memory operand.  A86 handles this the same way
it handles a MOV of segment registers: it generates a PUSH of the
source followed by a POP of the destination.
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Finally, A86 allows the XCHG of a segment register (except CS)
with any other word-sized quantity, as well as the XCHG of two
word-sized memory quantities.  If there is no machine instruction
available for XCHG a,b, then A86 generates PUSH a followed by MOV
a,b followed by POP b.


Local Symbols

If you examine most assembly language program symbol tables, you
will find that the symbols can be partitioned into two levels of
significance.   About half the symbols are the names of
procedures and variables having global significance.  If the
names of these symbols are chosen intelligently and carefully,
the program's readability improves drastically. (They usually
aren't chosen well, most often because the assembler restricts
symbols to 6 letters, or because the programmer's habits are
influenced by such assemblers.)

The other half of the symbols in a program have a much lower,
local significance.  They are only place markers used to
implement small loops and local branching (e.g., "skip the next 2
instructions if the Z-flag is set").  Assigning full-blown names
to these symbols reduces the readability of your program in two
ways:  First, it is harder to recognize local jumps for what they
are-- they are usually the assembly language equivalent of high
level language constructs like IF statements and WHILE loops.

Second, it is harder to follow the global, significant symbols
because they are buried in a sea of the place marker symbols in
the symbol table.

A86 solves this problem with local symbols.  If a symbol in your
program consists of a single letter followed by one or more
decimal digits (L3, X123, Y37, etc.), then the symbol is a local
symbol.  Local symbols do not appear in the A86 +X
cross-reference listing.  They can also be redefined to something
completely different later in the program.  Local symbols can be
of any type: labels, memory variables, etc.

Because local symbols can be redefined, you must take care to
specify which one you are referring to in your program.  If your
reference is a forward reference (the label occurs further down
in the program from the reference), then the reference must be
preceded by a ">".  For example,

L2:
  MOVSB
  INC BX
  LOOP L2    ; lack of ">" means L2 is above this statement
  .
  .
  JNZ >L2    ; ">" indicates L2 is below this statement
  .
  JMP >L2    ; JMP L2 is disallowed here: cannot overlap ranges
  .
L2:
                                                              5-4

I recommend that you assign all your local labels the names L0
through L9.  If your program is so complex that it needs more
than 10 place holders in any one stretch of code, then that
stretch needs to be rewritten.



Operands to AAM and AAD Instructions

Those of you who have examined 86 family opcodes with an eagle
eye will have noticed a somewhat spurious "0A" opcode generated
after every AAM or AAD instruction.  The opcode is there to
provide the constant divisor or multiplicand for the instruction.
Believe it or not, there wasn't enough room in the microcode of
the original 8086 to hold this constant!  Although Intel has
never announced the generality of AAM and AAD, it is there: you
can substitute any other constant for 0A (decimal 10), and that
constant will be used.  A86 supports this by letting you give a
constant byte-sized operand to AAM or AAD.  Particularly useful
are the instructions AAM 16, which unpacks AL into nibbles AH and
AL; and AAD 16, which reverses the process, packing nibbles AH
and AL into AL.

WARNING: A couple of my users point out to me that the AAD
instruction with a general operand won't work on the NEC V20 and
V30 chips.  The operand is assumed to be 10 no matter what it
really is.  Since a large number of PC "speed up" kits involve
switching to NEC chips, this will be seen on many PC's.  You
should not use AAD with an operand if you want your program to
run on everybody's machine.  Too bad.  AAM works fine, though.


Single-Operand Forms of the TEST Instruction

A86 allows the TEST instruction to have a single operand, to set
the flags according to the value of the operand.  If the operand
is a register, A86 generates a TEST of the register with itself.
If the operand is a memory quantity, A86 generates a TEST of the
memory with the constant -1 (i.e., the quantity will be ANDed
with an all 1's constant).  For example, instead of TEST DL,DL,
you can code simply TEST DL.  Instead of TEST WVAR,0FFFF, you can
code simply TEST WVAR.


Optimized LEA Instruction

Many assembly-language programmers are in the habit of using, for
example, LEA SI,MEMLOC instead of the equivalent MOV SI,OFFSET
MEMLOC to load an immediate value that represents the pointer to
a memory location.  However, the LEA instruction form generates
one more byte of object code than the MOV form. A86 recognizes
this situation and generates the more-efficient MOV instruction
when it can.  This also applies to register moves: MOV AX,BX
instead of LEA AX,[BX].
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I've gotten a little flak from some users about this feature.
They claim it violates my policy against "behind your back"
actions.  But I feel that this feature is completely equivalent
to code optimizations in other situations: the short JMP form
instead of the equivalent near JMP; a byte operand to ADD SI,4
instead of a word operand; the one-byte XCHG AX,BX instead of the
general XCHG rw,ew form; etc, etc, etc. In situations where there
is absolute functional equivalence between forms, A86 tries to
generate the most efficient form.  But for those who are not
convinced, I offer the +G2 switch, described in Chapter 3.

Some users have also gotten the mistaken impression, from reading
Intel's confusing specs, that the longer LEA is sometimes faster
than the shorter MOV.  This is never the case: those users are
reading the clock counts for the memory-fetch forms of MOV, not
the register-only or immediate-value forms.  If you don't believe
it, try timing 1000 consecutive LEA's in a loop that executes
50000 times, vs. a similar loop with the equivalent MOV.